If a vehicle is braking with active anti-locking mechanisms under differing traction conditions for the left and right vehicle wheels, where an individual anti-locking mechanism is provided for each wheel, a differential brake pressure level between the vehicle wheels with "good" traction and the vehicle wheels with "poor" traction. This brake pressure differential produces a yawing moment which can lead to a loss of control of the vehicle. This yawing moment can be avoided completely only by applying the so-called "select-low-method" to identify the brake pressure of that wheel that has the least traction as the command variable for control of the other wheels. In this manner, however, the braking distance would be increased, since only the areas with the poorer traction would be engaged, while the possible braking power of the wheels with the better traction would be "wasted"
Thus, in practice, brake pressures producing yawing moment have been permitted, but the possible differentials in brake pressure between the left and right front wheels have been limited to a predetermined value.
Limiting the pressure differentials presupposes, that the pressures actually prevailing in the brake cylinders are known. It is known, for example, that the brake cylinders of the right and left front wheels can be connected by a brake line in which a prestressed spring relief valve is inserted. When the difference between the two pressures exceeds the (sic) determined by the prestress limit of the spring, the relief valve opens so that the higher pressure is vented. This, solution, however is mechanically expensive.
It would also theoretically be possible to measure the respective brake pressures through the use of electro-mechanical pressure voltage transformers and then to adjust the permissible differential pressure through electronic means. But such a solution is also expensive because of the requisite measuring equipment. Moreover, such measuring equipment is susceptible to frequent breakdowns.